Advanced process control that comprises in-situ process monitoring and fault detection in semiconductor manufacturing is essential for reproducible production of complex structures. In typical etching and film deposition processes, the wafer parameters are measured using test wafers after the processing steps. If the measured parameters are not within the desired tolerances, the process parameters are adjusted and more test wafers are measured to assure process compliance. This post-process method is time consuming, inefficient, and expensive compared to real-time in-situ monitoring techniques.
These drawbacks reveal that real-time, in-situ process monitoring should be used whenever possible. The data that is acquired during a process step is used to improve the process by optimizing process conditions and detecting trends of departure from target values and early recognition of a possible catastrophic failure of the process equipment. In addition to etching and film deposition processes, chamber cleaning and chamber conditioning processes require in-situ monitoring.
Various spectroscopy methods have been established for real-time process monitoring. These methods include qualitative and quantitative analysis of the gaseous chemical species using techniques such as mass spectroscopy (MS) and optical emission spectroscopy (OES). These techniques provide information on the identity and concentration of gaseous species during the manufacturing process, which in turn can be correlated to various physical properties of the substrates.
Mass spectrometers are readily available instruments for detection, identification, and analysis of the components of a gaseous environment. They offer extreme sensitivity for detecting trace amounts of gaseous substances. In MS, the gaseous material is ionized through various techniques and the ions are then collected by the spectrometer. The ion signals are then translated into a mass spectrum that is used to identify what atoms or molecules are present in the gaseous environment.
Due to the relatively high pressure at the process monitoring point of a typical semiconductor process, the gas sampling in MS usually includes a pressure reduction system. The pressure reduction is carried out using a length of capillary tube or a throttle valve, and the mass spectrometer itself is pumped continuously. In this setup, it is possible that the sampling efficiency of the probe has mass dependence and extensive calibration can be necessary.
MS can provide a wealth of information on a process, yet the data are often complex and difficult to interpret. Complicated ion spectra can result from extensive fragmentation of the parent molecules in the mass spectrometer ionization region.
OES is a widely used method for process monitoring and control in semiconductor processing. OES is a non-invasive technique that has an extremely wide dynamic range and can perform process control (e.g., endpoint detection, etch rate, and partial pressure control) and diagnostics concurrently.
The point of monitoring the gaseous environment in semiconductor processes is frequently the process region since an excitation source such as plasma is needed to excite the gaseous species for analysis. A drawback to accessing the processing region for monitoring is that it can be invasive to the plasma and it is a more costly venture than to access regions downstream from the processing region. If the process monitoring point is downstream from the process zone, and therefore separate from the plasma, alternative means for exciting and/or ionizing the gaseous species are required.
The drawbacks for post-process analysis show that there is a need for real-time in-situ process monitoring of semiconductor processes using spectroscopic means that allow for comprehensive analysis of the gaseous environment. The analysis results are correlated to various physical properties of the substrates and can reduce the use of test wafer reiterative monitoring methods. Furthermore, there is a need for means of excitation of the gaseous environment that avoids fragmentation of the gaseous species. Also, there is a need for a monitoring point for process analysis that is non-invasive to the process region and is located downstream from the process zone.
The method of using metastable electronic energy transfer to excite gaseous species has previously been utilized in other types of semiconductor processes. Examples include selective dissociation of process gases for etching or deposition steps. Tokunaga in U.S. Pat. No. 5,874,013 entitled “Semiconductor integrated circuit arrangement fabrication method,” and Loewenstein in U.S. Pat. No. 5,002,632 entitled “Method and apparatus for etching semiconductor materials,” describe methods to obtain the desired etching species by dispersing an inert gas excited to a metastable state in an etching gas. Markunas in U.S. Pat. No. 5,180,435 titled “Remote plasma enhanced CVD method and apparatus for growing an epitaxial semiconductor layer,” describes a CVD apparatus for growing a layer on a substrate using metastable electronic energy transfer to partially dissociate and activate the CVD precursor gas. This leads to film deposition at lower temperatures than for purely pyrolytic processes.
In another example, Dodge in U.S. Pat. No. 4,309,187 entitled “Metastable energy transfer for analytical luminescence,” describes an alternative excitation method that uses a dielectric discharge to create metastable nitrogen species. The excitation method is shown to have a large dynamic range where concentrations as high as 1015 atoms/cc can be measured. The wavelength(s) and intensity of emitted light are used to determine the identity and the concentration of the species of interest.